Method and apparatus for reliably laser marking articles

ABSTRACT

The invention is a method and apparatus for laser marking a stainless steel specimen with commercially desirable marks. The method includes providing a laser processing system having a laser, and laser optics and a controller with pre-determined laser pulse parameters, selecting the pre-determined laser pulse parameters associated with the desired mark, and directing the laser marking system to produce laser pulses having laser pulse parameters associated with the desired marks including temporal pulse widths greater than about 1 and less than about 1000 picoseconds.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.12/823,895, filed on Jun. 25, 2010, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present invention relates to laser marking metal articles. Inparticular it relates to marking stainless steel with a laser processingsystem. More particularly it relates to marking stainless steel in adurable and commercially desirable fashion with a laser processingsystem. Specifically it relates to characterizing the interactionbetween infrared wavelength picosecond laser pulses and the stainlesssteel surface to identify laser parameters which will reliably andrepeatably create durable marks with a desired optical density.

BACKGROUND OF THE INVENTION

Marketed products commonly require some type of marking on the productfor commercial, regulatory, cosmetic or functional purposes. Desirableattributes for marking include consistent appearance, durability, andease of application. Appearance refers to the ability to reliably andrepeatably render a mark with a selected shape, color and opticaldensity. Durability is the quality of remaining unchanged in spite ofabrasion to the marked surface. Ease of application refers to the costin materials, time and resources of producing a mark includingprogrammability. Programmability refers to the ability to program themarking device with a new pattern to be marked by changing software asopposed to changing hardware such as screens or masks.

Stainless steel, which is strong, and has a durable surface finish, hasmany applications in industrial and commercial goods. Many articlesmanufactured out of metals such these as are in need of permanent,visible, commercially desirable marking. Stainless steel is an exemplarymaterial that has such needs. Metals such as stainless steel whichresist corrosion can be marked in this fashion. Marking stainless steelwith laser pulses produced by a laser processing system can make durablemarks quickly at extremely low cost per mark in a programmable fashion.

Creating color changes on the surface of stainless steel with laserpulses has been reported in the literature. One mechanism which has beenput forth to explain the change in optical density or color of metallicsurfaces is the creation of laser-induced periodic surface structures(LIPSS). The article “Colorizing metals with femtosecond laser pulses”by A. Y. Vorobyev and Chunlei Guo, Applied Physics Letters 92, (041914)2008, pp 41914-1 to 141914-3 describes various colors which may becreated on metals using femtosecond laser pulses. This article describesmaking black or gray marks on metal and creating a gold color on metal.Some other colors are mentioned but no further description is made.LIPSS is the only explanation offered for the creation of marks onmetallic surfaces. Further, only laser pulses having temporal pulsewidths of 65 femtoseconds are taught or suggested to create thesestructures.

Two articles discuss using picosecond laser pulses to create surfacechanges on semiconductor materials and metals. The articles SURFACERIPPLES ON SILICON AND GALLIUM ARSENIDE UNDER PICOSECOND LASERILLUMINATION, authors P.M. Fauchet and A. E. Siegman, Appl. Phys. Lett.40(9), 1 May 1982, pp 824-826, and GRADUAL SURFACE TRANSITIONS ONSEMICONDUCTORS INDUCED BY MULTIPLE PICOSECOND LASER PULSES, author P. M.Fauchet, Physics Letters, Vol. 93A, No. 3, 3 Jan. 1983 both describe indetail the changes that occur on semiconductor and metal surfaces whensubject to infrared and visible wavelength picosecond laser pulses.These articles describe how ripples form on the surface of thesematerials but do not discuss how the appearance of the material changesas a result of laser interaction.

Another problem with reliably and repeatably producing marks withdesired color and optical density in stainless steel is that the energyrequired to create very dark marks with readily available nanosecondpulse width solid state lasers is enough to cause damage to the metal,an undesirable result. “Darkness” or “lightness” or color names arerelative terms. A standard method of quantifying color is by referenceto the CIE system of colorimetry. This system is described in “CIEFundamentals for Color Measurements”, Ohno, Y., IS&T NIP16 Conf,Vancouver, Conn., Oct. 16-20, 2000, pp 540-545. In this system ofmeasurement, achieving a commercially desirable black mark requiresparameters less than or equal to L*=20, a*=+/−2, and b*=+/−2 on the CIEchromaticity scale. This results in a neutral colored black mark with novisible grayness or coloration.

What is desired but undisclosed by the art is a reliable and repeatablemethod of making commercially desirable black marks on stainless steelthat does not require an expensive femtosecond laser or ablate thesurface of the metal. What is needed then is a method for reliably andrepeatably creating marks having a desired optical density on stainlesssteel using a lower cost laser, without causing undesired damage to thesurface or requiring cleaning prior to anodization.

SUMMARY OF THE INVENTION

An embodiment of this invention creates a mark with desired propertieson a stainless steel specimen using a laser marking system. The lasermarking system has a laser which produces laser pulses having laserpulse parameters which control the laser fluence and laser beampositioning and therefore the laser dose to which the specimen isexposed. Laser parameters associated with the optimal marking range fora particular specimen are determined and stored in the laser processingsystem. The system is then directed to mark the specimen by directinglaser pulses to impinge the specimen using the stored laser pulseparameters to expose the specimen to laser doses within the optimalmarking range and thereby mark the specimen with commercially desirablemarks.

To achieve the foregoing with these and other aspects in accordance withthe purposes of the present invention, as embodied and broadly describedherein, a method for creating a visible mark with desirable commercialqualities on a stainless steel specimen and apparatus adapted to performthe method is disclosed herein. Included is a laser processing systemhaving a laser, laser optics, and motion stages all operativelyconnected to a controller with stored, predetermined laser pulseparameters. Stored laser pulse parameters associated with the desiredcolor and optical density mark are selected which direct the lasermarking system to produce laser pulses which expose the stainless steelspecimen to laser doses associated with the desired color and opticaldensity and thereby create marks with commercially desirable properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and b. Adapted laser marking system.

FIGS. 2a -f. SEM images of laser mark on stainless steel.

FIGS. 3a and b. Optical microscope images of laser mark on stainlesssteel.

FIGS. 4a -d. SEM images of laser mark on stainless steel.

FIG. 5. SEM image of laser mark on stainless steel.

FIG. 6. SEM image of laser mark on stainless steel.

FIG. 7. SEM image of laser mark on stainless steel.

FIG. 8. Laser fluence chart.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An embodiment of this invention marks stainless steel articles withdurable, visible marks with commercially desirable color and opticaldensity. This is done by using picosecond laser pulses with laserparameters predetermined to expose the area to be marked with a laserdose within a range that has been determined to create the marks withdesirable commercial properties. The picosecond laser pulses createcommercially desirable marks by altering the properties of the surfacewithout causing significant damage or adding material to the surface,thereby making the marks durable. Durable, commercially desirable marksare created on stainless steel by controlling the laser parameters whichcreate and direct picosecond laser pulses. One embodiment of thisinvention is a laser processing system adapted to produce laser pulseswith appropriate parameters in a programmable fashion. Exemplary laserpulse parameters which may be selected to improve the reliability andrepeatability of laser marking stainless steel include laser type,wavelength, pulse duration, pulse repletion rate, number of pulses,pulse energy, pulse temporal shape, pulse spatial shape, focal spot sizeand focal spot shape. Additional laser pulse parameters includespecifying the location of the focal spot relative to the surface of thearticle and in particular directing the relative motion of the laserpulses with respect to the article in coordination with the laser pulsesrepetition rate and timing to determine the spacing between successivepulses (bite size) and the spacing between parallel rows of laser pulses(pitch).

Laser pulse parameters are selected to control the total laser radiationdose delivered to the surface of the article to be marked. Producinglaser marks on stainless steel with commercially desirable properties isdependent upon the total laser radiation dose exposure. Laser radiationdose is defined as the total amount of laser radiation delivered to thesurface of the article being marked as measured in Joules. Measurementof the energy contained in a single laser pulse is relativelystraightforward, as is calculation of laser fluence in Joules/cm2 for asingle pulse. Due to complex geometries associated with typical markingschemes that require multiple overlapping laser pulses at each point,direct measurement or calculation of laser dose is very difficult. Laserradiation dose is a function of laser pulse fluence and laser pulsegeometry which includes properties such as spot size, focal distance,laser beam positioning, bite size and pitch. Both fluence and geometryaffect laser dose. Changing these parameters changes the dose of laserradiation which reaches a given location on the surface of the workpieceand thereby changes the appearance of the mark. Due to complexgeometries created by the overlaps in bite sizes and pitches smallerthan the radius of the spot size on the workpiece, measurement orcalculation of the actual dose delivered to a workpiece in practicalsituations can be difficult. Laser dose, for a given set of laser beampositioning parameters, is proportional to laser fluence. For thisreason, laser exposure is often discussed in terms of laser fluencerather than dose, with the understanding that the results areproportional. Typically in order to determine the effective laserexposure to use, a beginning laser fluence and set of laser positioningparameters is selected and then the correct dose is determinedexperimentally by varying the parameters associated with speed, bitesize, pitch and height of focal spot above the workpiece to determinethe optimal laser parameters to use for a given sample of stainlesssteel. If no reasonable set of laser positioning parameters results in adesirable mark, then the laser fluence can be adjusted and theexperiment repeated.

Embodiments of this invention create durable, commercially desirablemarks by darkening the surface of the stainless steal with opticaldensities which range from nearly undetectable with the unaided eye toblack depending upon the particular laser pulse parameters employed.Laser pulse parameters are determined to provide a particular range oflaser doses which are greater than the threshold for creating changes inthe surface but less than the threshold for creating large ripples ordamage. Exposing stainless steel to laser doses in this optimal markingrange creates uniform, dark, commercially desirable marks by creatingsmall, high frequency ripples within existing grain boundaries on thesurface of the stainless steel. Exposing stainless steel to laser dosesin excess of this optimal marking range creates larger, lower frequencyripples at right angles to the higher frequency ripples as reported inthe references that obliterate grain boundaries and create diffractioneffects. Laser marks created according to aspects of this invention areindicated by high frequency ripples and the continued presence of grainboundaries, with some lower frequency ripples possibly beginning to formbut not well enough organized to remove the grain boundaries.

Embodiments of this invention perform marking on stainless steel.Stainless steel used for this experiment was of type SAE grade 304-O,finished with a high polish finish that tends to remove grit associatedwith the initial finishing steps and leaves a highly reflective surface.Assuming that factors associated with laser positioning are heldconstant, the laser fluence F, defined by:F=E/awhere E is laser pulse energy in Joules and a is the area of the spotsize on the substrate in square centimeters, must be in the proper rangeto cause desired changes in the stainless steel surface. In order forthe fluence, F to be in the optimal marking range, it must satisfy therelation F_(u)<F<F_(r)<F_(s), where Fu is the laser modificationthreshold of the stainless steel substrate, where small, high frequencyripples begin to form within grain boundaries on the surface; F_(r) isthe threshold at which large, low frequency ripples begin to form andobliterate grain boundaries; and F_(s) is the damage threshold for thesurface layer where material begins being ablated from the surface.F_(u), F_(r) and F_(s) have been obtained experimentally and representthe fluence of the selected laser at which the substrate surface layerbegins to show signs of being modified by the laser energy (F_(u)),where the substrate surface layer becomes organized into large ripplesthat remove grain boundaries (F_(r)) and where damage that interfereswith the marking begins (F_(s)). For 10 picosecond (ps) IR pulses,exemplary values of F_(u) for stainless steel is about 47 mJ/cm², Fr isabout 62 mJ/cm² and Fs is about 73 mJ/cm². Using fluences in the optimalmarking range will produce commercially desirable marks at some speeds,bite sizes and pitches in the ranges listed in Table 1.

TABLE 1 Laser parameters Laser Type DPSS Nd:YVO4 Wavelength 1064 nmPulse duration 10 picoseconds Pulse temporal Gaussian Laser power 7 Wmax Rep Rate 200 KHz Polarization Linear Speed 300 mm/s Bite size 1.5microns Pitch 10 microns Spot size 130 microns Spot shape GaussianFluence .05 J/cm² Focal Height 4.6 mm +/− 0.1 mm step size

Marking stainless steel according to embodiments of this invention inthe fluence range indicated above appears to be a function of the metalgrain. Grain boundaries separate areas on the surface of the metal wherethe crystalline structure is generally homogeneous. It is known thatlaser/material interaction in this fluence range shows a dependence onpolarization. We observed, however, a previously unreported interactionbetween metallic crystals or grain, wherein the small ripples createdwithin grain boundaries seemed to show a dependence upon therelationship between grain direction within grain boundaries andpolarization, with some grain regions being more susceptible to laserradiation than other regions.

An embodiment of the instant invention uses an adapted laser processingsystem to mark stainless steel articles. An exemplary laser processingsystem which can be adapted to mark stainless steel articles is the ESIModel MM5330 laser micromachining system, manufactured by ElectroScientific Industries, Inc., Portland, Oreg. 97229. According to thespecifications found in the Model 5330 Service Guide, document no.147250-01a, May 2007, Electro Scientific Industries, Inc. Portland,Oreg. 97229, included herein by reference, this system is a lasermicromachining system employing a variety of diode-pumped Q-switchedsolid state lasers with an average power of up to 11 W at 90 K Hz pulserepetition rate at 355 nm UV wavelength. This laser may be adapted toproduce laser pulses with duration of about 1 ps to 1000 ps, orpreferably between 1 ps and 250 ps or more preferably between 10 ps and100 ps. The laser power can range from about 1 W to about 100 W,preferably between about 1 W and 50 W or more preferably between 5 W and25 W. These lasers operate at a rep rate of between about 1 KHz to about100 MHz, preferably between about 1 KHz and 1 MHz or more preferablybetween about 10 KHz and 100 KHz. The adapted laser system can directthe laser beam to move with respect to the specimen at speeds rangingfrom about 1 mm/s to about 1 m/s, preferably between about 50 mm/s andabout 500 mm/s, more preferably between about 100 mm/s and about 400mm/s The bite size or spacing between subsequent laser pulses on thesurface of the specimen can range from about 1 micron to about 1 mm,preferably between about 1 micron and about 500 microns, or morepreferably between about 1 micron and about 100 microns. The systemtypically operates with a pitch, or distance between adjacent lines oflaser pulse locations, of between about 1 micron to about 1 mm, orpreferably between about 1 micron and about 500 microns or morepreferably between about 1 micron and about 100 microns. The system isconfigured to focus the laser pulses down to a minimum focal spot sizeof between about 10 microns to about 500 microns, preferably betweenabout 50 microns and 250 microns, more preferably between about 100microns and about 200 microns. The system is configured to emit laserpulses with fluence of between about 0.01 J/cm² to about 100 J/cm²,preferably between about 0.1 J/cm² to about 25 J/cm², more preferablybetween about 0.1 J/cm² to about 10 J/cm². The system can also beconfigured to emit second harmonic doubled 532 nm wavelength pulses orthird harmonic 355 nm wavelength pulses. This system may be adapted bythe application of appropriate laser, laser optics, parts handlingequipment and control software to reliably and repeatably produce marksin stainless steel surfaces according to the methods disclosed herein.These adaptations permit the laser processing system to direct laserpulses with the appropriate laser parameters to the desired places on anappropriately positioned and held stainless steel article at the desiredrate and pitch to create the desired mark with desired color and opticaldensity. An embodiment of this invention comprises adaptations whichpermit the laser processing system to direct laser pulses with laserparameters as listed in Table 1. A diagram of such an adapted system isshown in FIGS. 1a and b.

FIG. 1a shows a diagram of an adapted ESI Model MM5330 lasermicromachining system 2 adapted for marking articles as an embodiment ofthe instant invention. Adaptations include a laser mirror and powerattenuator 4, a laser beam steering optics 6 and laser field optics 8adapted to handle the laser wavelength, power and beam sizes of thisembodiment, a chuck 10 adapted to fixture stainless steel specimens, acontroller 12 adapted to store and direct the system to emit laserpulses according to specifications in Table 1, a Y stage 14, an X stage18, and a Z stage (optics stage) 20 adapted to move the fixturedstainless steel article with respect to the laser beam focal spotaccording to the specifications in Table 1 and a camera 22 and viewingoptics 24 adapted to align and inspect the stainless steel specimen.

FIG. 1b shows another view of an adapted ESI Model MM5330 lasermicromachining system 2, including a laser interlock controller 26 thatcontrols the operation of the interlock sensors (not shown) whichprevent operation of the laser when various panels of the system areopened, controller 28, laser power supply 30, laser beam collimator 32,laser beam optics 34 and laser mirror 36, all of which have been adaptedto work with the adapted laser 38 which is a diode pumped Nd:YVO₄ solidstate laser operating at 1064 nm wavelength, model Rapid manufactured byLumera Laser GmbH, Kaiserslautern, Germany. The laser 38 is configuredto yield up to 6 W at a 2 MHz pulse repetition rate. The laser 38produces laser pulses with duration of 1 to 1,000 picoseconds incooperation with controller 28, and laser power supply 30. These laserpulses may be Gaussian or specially shaped by the laser beam optics 34.The laser optics 34, in cooperation with the controller 28, laser beamsteering optics 6 and laser field optics 8 cooperate to direct laserpulses to form a laser spot on a stainless steel specimen (not shown)fixtured by chuck 10. Motion control elements Y stage 14, X stage 18, Zstage (optics stage) 20 and laser beam steering optics 6 combine toprovide compound beam positioning capability, one aspect of which is theability to position the laser beam with respect to the specimen whilethe specimen is in continuous motion with respect to the laser beam.This capability is described in U.S. Pat. No. 5,751,585, inventorsDonald R. Cutler, Robert M. Pailthorp and Mark A. Unrath, issued May 12,1998 assigned to the assignee of this invention and which isincorporated herein by reference. Compound beam positioning includes theability to mark shapes on a specimen while the specimen is in relativemotion to the laser beam by having the controller 28 direct some portionof the motion control elements, namely Y stage 14, X stage 18, Z stage20 and laser beam steering optics 6 to compensate for continuousrelative motion induced by other portions of the motion controlelements.

The laser pulses (not shown) are also shaped by the laser beam optics 34in cooperation with controller 28. The laser beam optics 34 direct thelaser pulses' spatial shape, which may be Gaussian or specially shaped.For example, a “top hat” spatial profile may be used which delivers alaser pulse having an even distribution of fluence over the entire spotarea which impinges the article being marked. Specially shaped spatialprofiles such as this may be created using diffractive optical elementsor other optical elements. The laser spot size refers to the size of thefocal spot of the laser beam. The actual spot size on the surface of thespecimen being marked may be different due to the focal spot beingpositioned above or beneath the surface. In addition, the laser beamoptics 34, laser beam steering optics 6, laser field optics 8 and Zstage (optics stage) 20 cooperate to control the depth of focus of thelaser spot, or how quickly the spot goes out of focus as the point ofintersection on the specimen moves away from the focal plane. Bycontrolling the depth of focus, the controller 28 can direct the laserbeam optics 34, laser beam steering optics 6, laser field optics 8 and Zstage (optics stage) 20 to position the laser spot either at or near thesurface of the specimen repeatably with high precision. Making marks bypositioning the focal spot above or below the surface of the articleallows the laser beam to defocus by a specified amount and therebyincrease the area illuminated by the laser pulse and decrease the laserfluence at the surface. Since the geometry of the beam waist is known,precisely positioning the focal spot above or below the actual surfaceof the article will provide additional precision control over the spotsize and fluence. It was discovered that altering the laser fluence byaltering the laser spot geometry by positioning the focal spot combinedwith the use of picosecond lasers, which produce laser pulse widths inthe range from 1 to 1,000 picoseconds, is a way to reliably andrepeatably create marks on stainless steel. An advantage of usingpicosecond lasers is that they are much less expensive, require muchless maintenance, and typically have much longer operating lifetimesthan prior art femtosecond lasers.

FIGS. 2a through 2f are scanning electron micrographs of laser marks onstainless steel created according to an embodiment of this invention.FIGS. 2a-2f represent laser marks formed at fluences which range from 47mJ/cm² to 62 mJ/cm². In this case, fluence was altered by changing theheight of the focal spot above the workpiece. Changing the height of thefocal spot with respect to the surface of the workpiece causes the laserbeam to appear to de-focus at the surface, thereby spreading the laserenergy over a larger area and reducing the effective fluence at thesurface. Changing the height of the focal spot above or below thesurface is a rapid, reliable and precise way to adjust laser fluenceover the ranges desired by embodiments of this invention. Other methodsof altering the fluence include changing the pulse duration, changingthe laser power, changing the speed at which the laser beam is movingwith respect to the workpiece, changing the rep rate, or bite size.Changes in laser parameters which change the fluence or ultimately thedose delivered to the surface of the specimen will alter the appearanceof the resulting mark.

FIGS. 2a through 2f show marks created by altering the height of thefocal spot from about 5.5 mm above the surface of the article beingmarked down to about 4 5 mm above the surface of the article in 0.1 mmsteps. FIG. 2a shows a stainless steel article 40 marked with laserparameters as shown in Table 1 with the height of the laser spot abovethe workpiece set to 5.2 mm This image shows an area that has beenmarked with high spatial frequency ripples 42, along with an area thathas not been marked 44. Clearly shown is a boundary 46 between grainareas or crystallites on the surface of the stainless steel. Grainboundaries 46 are interfaces where grain areas with differentorientations meet. The overlaid scale 48 shows that the ripples have aperiod of about 10 nm and follow the grain boundaries 46. FIG. 2b showsanother portion of the same article 50 following marking with the samelaser parameters except that the laser spot height above the article tobe marked is reduced to 5.0 mm FIG. 2b shows high frequency ripples 52,with a period of about 10 nm as indicated by the embedded scale 54. Dueto the higher fluence, the ripples have spread more evenly over theexposed portion of the article. Note that grain boundaries 56 are stillclearly visible.

FIG. 2c shows another SEM image of an article marked according to anembodiment of this invention. This article 60 was marked using the laserparameters in Table 1, with the laser spot set 4.8 mm above the article.Clearly seen are high frequency ripples 62 with a period of about 10 nmas indicated by the scale 64. A grain boundary 66 is visible in thisimage. FIG. 2d shows another marked stainless steel article 70 markedaccording to laser parameters in Table 1, with the laser focal spot set4.6 mm above the article's surface. High frequency ripples 72, withperiod of about 10 nm cover the entire marked area, except for thebeginning of larger ripples 78 with spatial period of about 60-70 nm asindicated by the scale 74. Note that the grain boundaries 76 persist andthat the direction of the large ripples is perpendicular to thedirection of the high frequency ripples. The laser parameters listed inTable 1 and a laser spot height of 2.5 mm above the surface of astainless steel article produce commercially desirable marks withuniform appearance throughout wide viewing angles with optical densityequal to or less than about L*=20, a*=+/−2, and b*=+/−2 and representoptimal laser parameters for marking this particular sample of stainlesssteel according to an embodiment of this invention. Embodiments of thisinvention determine and use particular laser parameters which createrelatively small, high frequency ripples that preserve the grainstructure of the stainless steel while minimizing the creation oflarger, lower frequency ripples which form perpendicular to the smaller,high frequency ripples and obliterate the grain structure of the metalsurface. Use of these laser parameters to mark stainless steel yieldsuniform black marks with commercially desirable appearance and opticaldensity less than about L*=20, a*=+/−2, and b*=+/−2 as measured on theCIE chromaticity scale.

FIG. 2e shows a stainless steel article marked according to anembodiment of this invention with laser parameters as listed in Table 1and a laser spot height of 4.3 mm above the surface of the stainlesssteel article 80. This image shows the effect of increasing fluence onthe marks. At this fluence level, larger ripples 82 with period of about800 nm are forming as indicated by the scale 84. The larger ripples 82appear to form at right angles to the higher frequency ripples 42, 52,62 and have spatial periods almost 10 times greater than the higherfrequency ripples 42, 52, 62.

FIG. 2f shows a stainless steel article 90 having been exposed to laserpulses with laser parameters as listed in Table 1 with a focal spotheight of 4 mm above the surface. Note that large ripples 92 are wellorganized and are perpendicular to the direction of the smaller, higherfrequency ripples they replaced and have removed any indications ofgrain boundaries in the stainless steel. The scale 94 shows that theseripples have a period of about 800 nm The surface markings in FIGS. 2eand f have been exposed to laser fluences in excess Fr, the lowerfluence limit at which large ripples are organized on the surface of thestainless steel and do not represent optimal marks with desirablecommercial properties. Commercially desirable black is defined as a markhaving CIE chromaticity of L*=20, a*=+/−2, and b*=+/−2 or less.

FIG. 3a is an optical microscopic picture of a region of a stainlesssteel article 100 with a laser mark 102 created according to embodimentsof this invention. Laser parameters are as in Table 1 with a focal spotheight of 4.6 mm Note the clear presence of surface grain boundaries104. Magnification is represented by the scale 106. FIG. 3b is anoptical microscopic picture of a section 110 the same stainless steelarticle at a higher magnification 116, showing a mark 112 having grainboundaries 114 clearly visible.

FIG. 4a is a SEM image of a region of a stainless steel article 120marked according to an embodiment of this invention using laserparameters as shown in Table 1. This region 120 is marked with auniformly commercially desirable black color and exhibits clearlydefined grain boundaries 122. The magnification of this image is shownby the scale 124. FIG. 4b is an enlarged SEM image of a region of thesame marked stainless steel article 130 shown in FIG. 4a at highermagnification 138. This region 130 shows areas with small, highfrequency ripples 132 bordering areas where the small, high frequencyripples 132 are being partially organized into larger, low frequencyripples 134 at right angles to the high frequency ripples 132. Note thatthe grain boundaries 136 are generally visible. FIG. 4c is a SEM imageof a region 140 of the same marked stainless steel article from FIGS. 4aand 4b acquired at a higher magnification 148, showing areas of small,high frequency ripples 142, areas where the small, high frequencyripples are being replaced by larger, low frequency ripples 144, andclear grain boundaries in between 146. FIG. 4d is an enlarged SEM imageof a region 150 of the same marked stainless steel article shown inFIGS. 4 a, b, and c, acquired at magnification 154. This image shows aregion 150 of marked stainless steel with a surface texture 152 havingsmall, high frequency ripples running from lower left to upper right,being replaced by large, lower frequency ripples running from lowerright to upper left in the image. This process creates surface texture152 having a nodular texture as shown, which contributes to the lighttrapping properties of the surface and thereby its black appearance.These images represent optimal marking parameters according toembodiments of this invention.

FIG. 5 is a SEM image of a region of a stainless steel article 160marked according to an embodiment of this invention using laserparameters as shown in Table 1, acquired at a magnification indicated bythe scale 162. In this image, the laser parameters have been adjusted toapply a fluence F just greater than the minimum fluence Fu required tocause observable marking. Note the areas 164, 166, 168, 170 of small,high frequency ripples within defined grain boundaries 172. Also notethe differences between amplitude of the ripples between area 164 andareas 166, 168 and 170 indicating a relationship between laser pulsepolarization and crystal orientation in the metallic surface.

FIG. 6 is a SEM image of a region of a stainless steel article 200marked according to an embodiment of this invention using laserparameters as shown in Table 1, except that the laser fluence has beenadjusted to exceed the threshold at which large ripples begin to occur,F_(r), and acquired at a magnification indicated by the scale 202. Thisimage shows small, high frequency ripples 204 being replaced by large,low frequency ripples 206 at right angles to the former. Low frequencyripples, 206 are also obliterating grain boundaries 208. Also shown arecraters 210 caused by ablation damage to the surface. At these pointsthe laser fluence has exceeded the damage threshold F_(s). These fluencelevels result in marks with uneven appearance and limited viewingangles.

FIG. 7 is a SEM image of a region of a stainless steel article 212marked according to an embodiment of this invention using laserparameters as shown in Table 1, except that the laser fluence has beenadjusted to exceed the damage threshold of the stainless steel specimen,F_(s), acquired at a magnification indicated by the scale 214. In thisimage, the laser parameters have been adjusted to provide a fluence Fwhich greatly exceeds the damage threshold F_(s) of the stainless steelarticle. This image shows ablation damaged area 216 adjacent toundamaged surface 218. Marks created with this fluence show extensivedamage to the surface and irregular appearance.

Table 1 shows laser parameters used by an embodiment of this inventionto create commercially desirable marks on stainless steel. Laser typerefers to the technology used to generate laser pulses. In thisembodiment, a diode pumped, solid state Q-switched Nd:YVO₄ laser is usedto create laser pulses. Laser pulses which can be employed byembodiments of this invention can be created by other lasertechnologies, such as fiber lasers or combinations of solid state andfiber lasers. Wavelength refers to the wavelength of the laser pulses.Lasers typically produce substantially monochromatic pulses, where thewavelengths produced by the laser are all closely grouped around asingle wavelength, in this case 1064 nm Laser wavelengths can be alteredby harmonic frequency generation, where non-linear crystals are used todouble or triple the frequency of the laser, making the output pulseshave wavelengths of 532 nm or 355 nm respectively. Frequency doubled ortripled laser pulses can be used by embodiments of this invention. Thepulse duration refers to the temporal distribution of energy in thepulse. As mentioned above, pulse duration may be measured by a simplefull width at half maximum (FWHM) measure for simple shapes such asGaussian, or by the integral square method for more complex shapes. Thelaser power is measured by summing the power emitted by the laser over aperiod of time which encompasses several pulses and averaging the powerto get a value for average laser power. Repetition rate (rep rate)refers to the rate at which laser pulses are emitted by the laser.Typically lasers exhibit an inverse relationship between laser rep rateand laser power, where the faster the pulse repetition rate, the lowerthe laser power per pulse.

Laser speed refers to the rate at which the laser beam moves withrespect to the workpiece. We define the laser beam as the path that thelaser pulses travel as they are emitted from the laser through the laseroptics to the workpiece. An embodiment of this invention uses motioncontrol stages upon which the article to be marked is fixtured incooperation with laser beam steering optics to direct the laser beam tothe workpiece in such a fashion as to cause the laser beam to moverelative to the workpiece as the laser is pulsed. Bite size refers tothe distance between successive laser pulses as measured on the surfaceof the workpiece. Bite size is a function of rep rate and laser speed.

An embodiment of the instant invention performs marking on stainlesssteel. For these marking to happen, the laser fluence must satisfyF_(u)<F<F_(r)<F_(s), where F_(u) is the laser modification threshold ofthe stainless steel substrate, F_(r) is the fluence at which large, lowfrequency ripples begin to replace small, high frequency ripples, andF_(s) is the damaging threshold for the surface layer. F_(u), F_(r) andF_(s) have been obtained by experiments and represents the fluence ofthe selected laser at which the substrate surface layer begins to showsigns of being modified by the laser energy (F_(u)), where the largeripples start (F_(r)) and where damage that interferes with the markingbegins (F_(s)). For 10 ps IR pulses, our experiments show that F_(u) forstainless steel is about 47 mJ/cm², F_(r) is about 62 mJ/cm² and F_(s)is about 73 mJ/cm². FIG. 8 is a chart 220 showing the relationshipbetween laser fluence 222, non-marking 224, commercially desirablemarking 226, non-desirable marking 228, and damage 230. Because of thedifficulties in calculating and measuring dose, fluence is used as anindicating parameter, assuming that bite size and pitch are heldconstant. Holding bite size and pitch constant makes dose proportionalto fluence. Different pulse durations and laser wavelengths, forexample, would each have corresponding values of F_(u), F_(r) and F_(s),which would have to be determined prior to processing. The actualthresholds to be used for these laser parameters may be determinedexperimentally. In the same fashion, different samples of stainlesssteel materials will react differently to the laser pulses, thereforethe precise laser parameters to be used for a particular stainless steelarticle will be determined prior to processing.

Laser marking of stainless steel can also be achieved by an embodimentof the instant invention which uses picosecond IR wavelength laserpulses to mark the surface. This aspect creates marks of varyinggrayscale densities by varying the laser fluence at the surface of thestainless steel in at least two different manners. As discussed above,varying the fluence at the surface can be achieved by positioning thefocal spot above or below the surface of the stainless steel. A secondmanner of controlling grey scale is to vary the total dose at thesurface of the stainless steel by changing the bite sizes or linepitches when marking the desired patterns. Changing bite sizes refers toadjusting the relative position between successive pulses as the pulsesare delivered as the laser beam is in relative motion to the article.Bite size can be adjusted by changing the laser repetition rate, therate of relative motion between the laser beam and the article or both.Varying line pitches refers to adjusting the distance between markedlines to achieve various degrees of overlapping as the laser beam isscanned along adjacent lines. Fluence may also be varied by varyinglaser power, laser pulse duration or spot size among other laserparameters

It will be apparent to those of ordinary skill in the art that manychanges may be made to the details of the above-described embodiments ofthis invention without departing from the underlying principles thereof.The scope of the present invention should, therefore, be determined onlyby the following claims.

We claim:
 1. A method for marking an article including a stainless steelsurface, the method comprising: impinging a region of the stainlesssteel surface with a spot area of a laser beam to modify a morphology ofthe region of the metallic surface to form a modified surface regionhaving a visual appearance different from another region of thestainless steel surface outside the modified surface region, whereinimpinging the region employs laser pulse parameters that comprise pulsewidth, wherein the pulse width ranges from about 1 picosecond to about1000picoseconds, wherein the laser pulse parameters comprise pulsefluence F, wherein the pulse fluence F is in an optimal marking rangeand satisfies a relationship F_(u)<F<F_(r)<F_(s), wherein F_(u) is alaser modification threshold of the stainless steel surface at whichsmall high frequency ripples begin to form within grain boundaries onthe stainless steel surface, wherein F_(r) is the threshold at whichlarge low frequency ripples begin to form and obliterate grainboundaries, herein F_(s) is the damage threshold for the surface atwhich material begins being ablated from the stainless steel surface,wherein F is less than 0.1 J/cm², wherein the modified surface regionincludes a first plurality of ripples and a second plurality of ripples,wherein a characteristic of the first plurality of ripples is differentfrom a corresponding characteristic of the second plurality of ripples,and wherein the modified surface region exhibits a desired opticaldensity that is equal to or less than L*=20, a*=+/−2, and b*=+/−2 on aCIE chromaticity scale.
 2. The method of claim 1, wherein a 1-micronsquare area of the region includes multiple complete periods of thefirst or second ripples.
 3. The method of claim 1, wherein the rippleshave a spatial period of less than 60 nm.
 4. The method of claim 1,wherein the ripples have a spatial period of between 6 and 10 nm.
 5. Themethod of claim 1, wherein a 0.5-micron square area of the regionincludes multiple complete periods of the first or second ripples. 6.The method of claim 1, wherein the pulse fluence F is greater than thefluence F_(u), but at a minimum fluence required to cause observablemarking.
 7. A method for creating a mark on a stainless steel specimenwherein the metal specimen has a fluence threshold F_(s) above which thestainless steel specimen tends to become ablated, the method comprising:providing a laser marking system configured to direct laser pulses ontothe stainless steel specimen at a controllable pulse fluence F, saidlaser marking system having stored thereon laser pulse parameters,wherein said laser pulse parameters comprise pulse width, wherein saidpulse width ranges from about 1 picosecond to about 1000 picoseconds,wherein said laser pulse parameters comprise pulse fluence F, whereinsaid fluence F is in an optimal marking range and satisfies arelationship F_(u)<F<F_(r)<F_(s), wherein F_(u) is a laser modificationthreshold of the stainless steel specimen at which small high frequencyripples begin to form within grain boundaries on its surface, wherein Fis less than 0.1 J/cm², wherein F_(r) is the threshold at which largelow frequency ripples begin to form and obliterate grain boundaries, andwherein F_(s) is the damage threshold for the surface at which materialbegins being ablated from the surface; and controlling said lasermarking system according to said stored laser pulse parameters to directlaser pulses onto the stainless steel specimen at a pulse fluence belowsaid fluence threshold thereby creating said mark.